The primary reasons for using wellhead separators are to prevent wear and blockage of choke valves, to prevent wear and subsequent rupture of piping, to prevent damage and malfunction of instruments, and to prevent vessels filling with the particulate materials. The wellhead separation duty can include the requirement to separate particles up to 12 to 25 mm in size, and to separate hard abrasive particles. Debris from the perforation of the well casing in the production zone is a common source of large particulate material, and the production of surplus ceramic or garnet propant from well workovers is also a common source of abrasive particulate material.
In wellhead hydrocyclone separators it is common to separate the pressure containing function from the separation function, i.e. to put hydrocyclones in a pressure vessel, because the hydrocyclone will suffer wear, and if the hydrocyclone is itself pressure containing the wear will eventually compromise its pressure containing ability. The very high pressures which may occur at a well head, typically up to 1,400 bar (20,000 pounds per square inch) imposes a practical limit on the diameter of the vessel which may be built to contain the one or more hydrocyclone separators which perform the separation.
Well head hydrocyclone separators may contain a single large hydrocyclone of, say, 300 to 400 mm internal diameter or a number of smaller hydrocyclones of typically around 75 mm (3 inches) internal diameter. The smaller hydrocyclones provide a better separation of small particles than the larger hydrocyclone, whilst in a given diameter of pressure containment vessel a larger hydrocyclone can be designed to pass a higher flowrate than the assembly of small cyclones that could be fitted in the pressure vessel. However, the inlets of the smaller hydrocyclones may not be large enough to pass the 12 to 25 mm particles, whereas the larger hydrocyclone can easily do so.
The disadvantage of larger hydrocyclones is that they cannot be made in such hard wearing materials as small cyclones. Small cyclones for wellhead separators are made from isostatically pressed ceramic powders, for example Aluminas or Sialons, which can achieve hardnesses up to 2000 Hv. The largest parts that can be made by this process have a finished diameter between 200 and 300 mm diameter which is not large enough for the manufacture of a large hycrocyclone. Larger ceramic parts can be made by the reaction bonding process. Reaction bonded ceramic tends to be anisotropic and to contain a large proportion of voids which much reduce its strength and wear properties in comparison to isostatically pressed ceramic. This material has been used to make parts for large hydrocyclones for mineral mining applications, but it has not been widely used in high pressure well head hydrocyclone separators. Instead, larger cyclones for higher pressure applications have tended to be made from ductile and machinable materials so that they can be formed into the hydrocyclone shape. These materials may have hardness's typically between 250 Hv and 400 Hv, but after forming they may have linings of harder materials applied to their interior surfaces, some linings claiming hardness's near to what is achieved in the isostatically pressed ceramics. In service, however, large cyclones constructed as described have proved to be vastly inferior in terms of wear life to small cyclones constructed from isostatically pressed ceramic.
It is common for a large hydrocyclone to have a substantially tangential inlet that is aligned with and fed directly from the inlet port of the separator vessel. As a consequence, all particles tend to be travelling at the same velocity as the incoming fluid, and hence large particles, of e.g. 12-25 mm diameter will have a correspondingly large amount of kinetic energy which will rapidly wear the cylindrical wall of the hydrocyclone, where they first impinge on it.
In separators with a number of small hydrocyclones the fluids are introduced into an inlet chamber of the vessel where they decelerate and must change direction before entering the hydrocyclones. Following this deceleration and change of direction they therefore approach the inlets of the smaller cyclones with a lower velocity and therefore the large particles have a relatively lower kinetic energy as compared to those entering larger hydrocyclones. It is the purpose of the hydrocyclone inlet to accelerate the flow entering the cylindrical section of the hydrocyclone tube to a high velocity, but it is thought that the length of the inlet duct of a small hydrocyclone may be insufficient to allow acceleration of large particles to the same velocity as the fluid is particularly where the fluid is gas or largely gas and is therefore of low density. In separators with a number of small hydrocyclones it is also known that large particles may drop to the bottom of the inlet chamber because the decelerated flow velocity is insufficient to suspend them.
The methods of mounting both the large and the small hydrocyclones in a vessel require a full diameter mechanical joint across the section of the vessel where the hydrocyclones are mounted. This joint is required to be openable and resealable to allow the hydrocyclone tubes to be inspected for wear and replaced.
At the high pressures that the wellhead hydrocyclone separators are required to operate at, the practical limits of conventional technology to provide a suitable joint are being approached. For example, for a vessel with a 600 mm internal diameter and a design pressure of 860 barg it is virtually impossible to make a flanged joint which will hold enough bolts to resist the axial pressure force through the joint, and the large size of the flange becomes an impediment to the function of the separator. Instead, this problem may be solved by means of an annular clamp arrangement having internally disposed cam surfaces which cooperate with correspondingly-shaped surfaces on the outer rim of the respective flanges which are then pinched together by bolts so that, effectively, the jaws of the clamp bear against the entire periphery of the flanges in this region, thereby making for a much stronger and more compact connection than is possible through the use of radially disposed nuts and bolts conventionally used for connecting flanged halves of small bore separator housings together.
The present invention is derived in part from the realisation that whilst it is preferable to utilise single large diameter cyclone separator assemblies in high pressure applications as compared to multiple small bore assemblies to allow higher flow rates through such separators, it would be preferable to arrange for the cyclone tube itself to be segmented.
The present invention is provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior inventions of this type. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.